Thanks all, I’ve found the solution.
Switched output setting in GX-100 to Line/Phones instead of Tube Combo 1x12, put the S/R block in the right spot, eliminated Amp block on signal chain and that did the trick, got my pre-amp back and regular Marshall tone.
Thanks a bunch for all the input guys, much appreciated!!
I was precisely about to ask what was the output mode selected... and to add that each time I use a MFX in 4CM, I set in on Direct / Line / Phones / studio mode, with digital preamps & cabs obviously disabled. It's normally the flatest output setting in a MFX.
Some recent modelers tend to have embedded EQing curves affecting the tone with other output modes, even when all FX are disabled (the Nux MG30 being an example of that, with a Line/phones option not even flat itself). I must admit that I don't understand this choice and I'll underline that it's not always easy to solve with an onboard EQ...
Anyway: enjoy with your JCM & GX!